Arthur/ssh check

Check meant to:
1. Check the status of SFTP
2.Check The response time 


@LeoCavaille 
Nice.
You'll also have to add that dependency to the omnibus build:
https://github.com/DataDog/dd-agent-omnibus/blob/master/config/projects/datadog-agent.rb#L72

and create a software definition for it similar to that:
https://github.com/DataDog/omnibus-software/blob/master/config/software/pymongo.rb